1. Field of the Invention
The present invention relates to a sizing agent for quartz glass fibers, a quartz glass fiber or a quartz glass yarn having applied thereonto the sizing agent, a quartz glass cloth, a prepreg for a printed circuit board, and a product using the quartz glass fiber.
2. Description of the Related Art
An E-glass cloth, a D-glass cloth, and the like obtained by weaving E-glass fibers, D-glass fibers, and the like have hitherto been used as a glass cloth to be used in a multi-layer printed circuit board.
However, in recent years, along with reductions in weight, thickness, length, and size, and multifunctionalization of high-performance mobile terminals, such as a smartphone and a tablet PC, the multi-layer printed circuit board, on which various electronic components are installed, has been required to adopt high density interconnection, have excellent high frequency characteristics, be highly multi-layered, and be reduced in thickness. With such background, low thermal expansion, a low dielectric constant, and thinning are strongly demanded for the glass cloth serving as a base material constituting the printed circuit board.
Therefore, among glass fibers, quartz glass fibers having a low linear expansion coefficient and having a low dielectric constant and dielectric tangent have attracted attention. A quartz glass cloth using the quartz glass fibers is specifically required to have a thickness of 20 μm or less.
In general, the glass cloth is obtained by weaving glass fibers having applied thereonto a sizing agent containing starch as a main component of a coating forming agent. As a loom to be used in the weaving, an air jet loom configured to jet weft with air is generally used from the viewpoints of productivity and dimensional stability of the cloth.
However, in the weaving with the air jet loom, there is a problem that fluff is liable to be generated on the glass cloth because a glass fiber bundle is jetted at significantly high speed. The fluff on a surface of the glass cloth is unfavorable because, in the printed circuit board manufactured by laminating a thin copper foil or the like on a prepreg subjected to provisional curing through impregnation of the glass cloth with a resin, the fluff has a risk of damaging the copper foil or the like and causes an electrical insulation failure. Therefore, it is necessary that the amount of the fluff on the surface of the glass cloth be significantly small.
A cause for the generation of the fluff on the surface of the glass cloth is, for example, chargeability of the glass fibers, and particularly in the case of quartz glass, the generation of the fluff is remarkable. For example, multicomponent glass, such as E-glass, which is used as a versatile product, has electrical conductivity in a molten state, and besides, is spun from a platinum-based nozzle at the time of spinning. Therefore, the multicomponent glass is relatively less liable to be charged. In addition, it is obvious that also alkali-free glass containing 0.2 wt % or less of an alkali metal oxide (Li2O, Na2O, or K2O) has electrical conductivity in a molten state also from the fact that a melting method using current application is utilized in Patent Document 1.
Meanwhile, the quartz glass does not contain any ion conductive species, such as alkali metals, and hence has low electrical conductivity even in a molten and softened state. Therefore, the quartz glass is liable to be charged. In actuality, while the quartz glass is spun by a known method as disclosed in Patent Documents 2-4, the quartz glass fibers are highly negatively charged at a charge potential of −2.0 kV or more at the time of spinning owing to friction with a flame stream, flow friction in the glass, and the like.
In order to suppress the fluff caused by static electricity in a production process for the glass fibers and a weaving process for the glass cloth, various sizing agents for glass fibers have hitherto been investigated. For example, a sizing agent for glass fibers disclosed in Patent Document 5 uses, as an antistatic agent, a cationic, non-ionic, anionic, or amphoteric surfactant. However, the sizing agent has a poor antistatic effect, and hence the antistatic agent is applied onto a glass fiber bundle again after the sizing agent for glass fibers is applied onto the glass fiber bundle and dried. In this case, the application of the sizing agent for glass fibers in two stages is troublesome and increases production cost. In addition, sizing agents for glass fibers disclosed in Patent Documents 6-7 have added thereto, as antistatic agents, octyldimethylammonium ethosulfate and a quaternary ammonium salt, such as lauryltrimethylammonium chloride or an alkylbenzyldimethylammonium chloride. However, those antistatic agents are liable to cause migration, and hence are not uniformly applied onto the surfaces of glass fibers, with the result that uneven adhesion of the antistatic agents is caused in the length direction of the glass fibers. The sizing agents even in such state are effective for glass fibers which are relatively less liable to be charged, such as E-glass fibers. However, when such sizing agent is applied to the quartz glass, an antistatic effect is not obtained sufficiently, and hence the quartz glass fibers are liable to be separated from each other, and the slip property of the quartz glass fibers lowers owing to, for example, their bonding to a loom jig, with the result that the generation of the fluff at the time of weaving cannot be suppressed.
Other than the addition of the antistatic agents described above, there has been an attempt to suppress the generation of static electricity through cationization of a coating forming agent. For example, in Patent Document 8, a dispersant formed of a linear cationic copolymer containing 65 mol % to 99 mol % of an ethylene structural unit and 1 mol % to 35 mol % of an acrylamide structural unit, and having a weight average molecular weight of 1,000 to 50,000 is added, to thereby control chargeability and thus suppress the fluff. However, acrylamide is classified as “a substance probably carcinogenic to humans (Group 2A)” in International Agency for Research on Cancer, and is not preferred from the viewpoint of health. In addition, in Patent Document 9, a copolymer emulsion obtained through emulsion polymerization of a vinyl acetate monomer and the like in the presence of a non-ionic surfactant alone or in the presence of the non-ionic surfactant and a cationic surfactant is added, to thereby enhance conformity with surfaces of the glass fibers and thus improve a bundling property. However, unwinding resistance between the respective glass strands is large, and the fluff cannot be suppressed. Further, a glass yarn does not successfully ride air owing to an excessively good bundling property, and weaving with the air jet loom is difficult. Meanwhile, in Patent Document 10, gelatinized starch is added in addition to an ethylene-vinyl acetate copolymer and a vinyl acetate polymer in order to lower the unwinding resistance between the respective glass strands. However, an antistatic effect is poor, and a quaternary ammonium salt is added.
Meanwhile, in Patent Document 11, there is disclosed that, while the charge potentials of a glass fiber bundle and glass fiber fabric in a weaving process are ideally controlled to ±0 kV, these charge potentials generally tend to largely change to a negative side with changes in temperature and humidity, and hence stable operation can be achieved when the center values of the charge potentials are controlled in advance to a slightly positive side, more specifically, to from +1.5 kV to −1.0 kV.
However, as described above, the quartz glass fibers are liable to be highly negatively charged, and hence it is difficult to keep the charge potential within a range of from +1.5 kV to −1.0 kV with the conventional sizing agents for glass fibers. As a result, the generation of the fluff on the surface of the glass cloth cannot be suppressed.